Metal nanoparticles, particles of elemental metal in pure or alloyed form with a dimension less than 100 nm, have unique physical, chemical, electrical, magnetic, optical, and other properties in comparison to their corresponding bulk metals. As such they are in use or under development in fields such as chemistry, medicine, energy, and advanced electronics, among others.
Synthetic methods for metallic nanoparticles are typically characterized as being “top-down” or “bottom-up” and comprise a variety of chemical, physical, and even biological approaches. Top-down techniques involve the physical breakdown of macroscale or bulk metals into nanoscale particles using a variety of physical forces. Bottom-up methods involve the formation of nanoparticles from isolated atoms, molecules, or clusters.
Physical force methods for top-down metal nanoparticle synthesis have included milling of macroscale metal particles, laser ablation of mascroscale metals, and spark erosion of macroscale metals. Chemical approaches to bottom-up synthesis commonly involve the reduction of metal salt to zero-valent metal coupled with growth around nucleation seed particles or self-nucleation and growth into metal nanoparticles.
While each of these methods can be effective in certain circumstances each also has disadvantages or situational inapplicability. Direct milling methods can be limited in the size of particles obtainable (production of particles smaller than 20 nm is often difficult) and can lead to loss of control of the stoichiometric ratios of alloys. Other physical methods can be expensive or otherwise unamenable to industrial scale.
Chemical reduction techniques can fail in situations where the metal cation is resistant to reduction. Mn(II) for example is notoriously impervious to chemical reduction. Conventional chemical reduction approaches can also be unsuitable for producing nanoparticles for applications that are highly sensitive to oxidation. Tin nanoparticles, for example, can be difficult to obtain from reduction approaches at sizes less than 20 nm and even when so obtained tend to contain a large proportion of SnO2.
Tin is a promising material for battery electrodes. For example, as an anode in a Li-ion battery, tin can store approximately three times the charge density of the commonly used graphite anode. Recently it has been shown that tin-based material holds great promise in use as a Mg-ion insertion type anode for high energy density Mg-ion batteries. In particular, anode material fabricated from 100 nm tin powder achieved high capacity and low insertion/extraction voltage.
Upon magnesiation of bismuth, which can occur during operation of a Mg-ion battery having a bismuth-based anode, a super-ionic conductive material, Mg3Bi2, is believed to form. In contrast, magnesiated tin does not form a super-ionic conductive material and as mentioned has susceptibility toward poor rate capability. An anodic active material which incorporates the beneficial properties of both tin and bismuth, such as tin-bismuth core-shell nanoparticles, can have the ability to improve the performance of electrochemical cells in general and of Mg-ion electrochemical cells in particular.